Reducing agents metal hydride reagents

Nitriles are reduced with metal hydride reagents to form either 1 ° amines or aldehydes, depending on the reducing agent. 

Although -3-pinanyl-9-borabicyclo[3.3.1]nonane and related substances have also been developed as efficient asymmetric reducing agents for carbonyl compounds (Volume 8, Chapter 1.3), we discuss here only asymmetric reductions using chirally modified metal hydride reagents. The asymmetric hydrosilyl-ation of a carbonyl group catalyzed by a chirally modified transition metal is mentioned briefly. 

Carboxylic acids and esters frequently must be protected against the attack of organometallic reagents, e.g. metal alkyls and hydrides, and reducing agents like LiAlH. For this purpose they usually are converted to orthoesters, oxazolines or oxazoles. 

Silylated 1,4-cyclohexadienes were used to replace tin hydride as reducing agents in radical chain reactions. They were prepared by TBDMSCl interception of the regioselec-tive metallation intermediate of cyclohexadienes, and subsequent methylation (eq 40). The 3-TBDMS-2,4-dimethoxy-l,4-cyclohexadiene was found to be a very efficient hydrogen donor reagent in various radical reactions. 

For most laboratory scale reductions of aldehydes and ketones catalytic hydro genation has been replaced by methods based on metal hydride reducing agents The two most common reagents are sodium borohydride and lithium aluminum hydride... 

Sulphoxides are reduced by the more powerful metal hydride reducing agents, but the less reactive reagents such as sodium borohydride are ineffective. Recently, Yoon has... 

However, lithinm aluminum hydride or zinc metal and HCl (5) are required as reducing agents to reduce the thiocyanate to the thiol. These reducing agents are stoichiometric reagents and aren t environmentally acceptable at this time because of their hazardous properties and waste disposal problems on a large manufacturing scale. 

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